The present invention relates to an optical system to be employed in an optical disc drive to read/write data from/to an optical disc.
Recently, technology in the field of magneto-optical disc drives has been greatly improved such that a data recording density on a magneto-optical disc has reached in excess of 10 Gbits/inch.sup.2.
In such an optical disc drive, an objective optical system is mounted on an arm which is movable in a transverse direction of tracks formed on an optical disc for rough tracking. Firstly, the rough tracking is performed to locate the optical head in the vicinity of the track. Then, an incident angle of a beam incident on the objective optical system is changed to locate a spot of beam formed by the objective optical system for fine tracking, with use of a galvano mirror or the like. During the fine tracking operation, the beam spot is accurately located on one of the tracks whose pitch is, for example, 0.34 .mu.m. When the beam spot is moved on the disc surface, i.e., when the incident angle of the beam incident on the objective optical system is being changed, it is preferable that an intensity distribution of the beam does not vary on the disc surface. In other words, it is preferable that, in such an optical system, a coupling efficiency of a light beam traveling between the objective optical system and a light source applicable to the optical disc drive is relatively high.